The invention relates to optical articles including holographic recording media, in particular media useful either with holographic storage systems or as components such as optical filters or beam steerers. In particular, this invention relates to holographic recording compositions that are self-sealing and therefore moisture resistant and environmentally durable.
Developers of information storage devices and methods continue to seek increased storage capacity. As part of this development, so-called page-wise memory systems, in particular, a holographic system, have been suggested as alternatives to conventional memory devices.
A hologram stores data in three dimensions and reads an entire page of data at one time, i.e., page-wise, which is unlike an optical CD disk that stores data in two dimensions and reads a track at a time. Page-wise systems involve the storage and readout of an entire two-dimensional representation, e.g., a page, of data. Typically, recording light passes through a two-dimensional array of dark and transparent areas representing data, and the holographic system stores, in three dimensions, holographic representations of the pages as patterns of varying refractive index imprinted into a storage medium. Holographic systems are discussed generally in D. Psaltis et al., xe2x80x9cHolographic Memories,xe2x80x9d Scientific American, Nov. 1995, the disclosure of which is hereby incorporated by reference. One method of holographic storage is phase correlation multiplex holography, which is described in U.S. Pat. No. 5,719,691 issued Feb. 17, 1998, the disclosure of which is hereby incorporated by reference.
The advantages of recording a hologram are high density (storage of hundreds of billions of bytes of data), high speed (transfer rate of a billion or more bits per second) and ability to select a randomly chosen data element in 100 microseconds or less. These advantages arise from three-dimensional recording and from simultaneous readout of an entire page of data at one time.
A hologram is a pattern, also known as a grating, which is formed when two laser beams interfere with each other in a light-sensitive material (LSM) whose optical properties are altered by the intersecting beams. One choice of a LSM is a photosensitive polymer film. See, e.g., W. K. Smothers et al., xe2x80x9cPhotopolymers for Holography,xe2x80x9d SPIE OE/Laser Conference, 1212-03, Los Angeles, Calif., 1990. The material described in this article contains a photoimageable system containing a liquid monomer material (a photoactive monomer) and a photoinitiator (which promotes the polymerization of the monomer upon exposure to light), where the photoimageable system is in an organic polymer host matrix that is substantially inert to the exposure light. During writing of information into the material (by passing recording light through an array representing data), the monomer polymerizes in the exposed regions.
U.S. Pat. No. 6,103,454 and application Ser. No. 09/046,822, the disclosures of which are hereby incorporated by reference, also relate to a photoimageable system in an organic polymer matrix. In particular, the application discloses a recording medium formed by polymerizing matrix material in situ from a fluid mixture of organic oligomer matrix precursor and a photoimageable system.
The conventional photoimageable holographic media having a crosslinked matrix formed by the reaction of polyisocyanates and polyols have been optimized for a matrix composition formed by the reaction of stoichiometric amounts of polyisocyanates to polyols. However, applicants unexpectedly found that a cross-linked matrix formed by the reaction of a composition containing an excess amount (i.e. non-stoichiometric amount) of polyisocyanates to polyols, once formed into holographic recording articles, exhibit high optical clarity, and low scattering.
This invention in high performance holographic recording articles is based on novel holographic articles containing crosslinked matrix formed from formulations containing an excess amount (i.e. non-stoichiometric amount) of polyisocyanates to polyols, which provide several advantages to the holographic performance of these media.
Applicants further found that when the polyols contain compounds such as fluorine, chlorine and silicon, the articles formed would also be optically clear, low scattering, and free of any the cloudiness and incompatibility commonly found in hydrocarbon systems mixing with the aforementioned fluorine, chlorine and silicon compounds. In the articles, the excess amount of polyisocyanates in the regions exposed to atmosphere or any moisture containing environment would react over time with the moisture to form a hard polyurea barrier.
Additional advantages of this invention would become readily apparent to those skilled in this art from the following detailed description, wherein only the preferred embodiments of this invention are shown and described, simply by way of illustration of the best mode contemplated for carrying out this invention. As would be realized, this invention is capable of other and different embodiments, and its details are capable of modifications in various obvious respects, all without departing from this invention. Accordingly, the drawings and description are to be regarded as illustrative in nature and not as restrictive.